Do not use geometry and memoir together, as they will fight for use of lengths. Memoir has all you need. Also keep in mind that stocksize is the whole PDF area, and trimmedsize is the finished area. Sometimes trimmed is smaller than stock. If so, you probably need to center trim within stock, using trimtop and trimedge. Margins are always measured relative to trim, not stock.
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RobtAJun 13 at 18:58

1 Answer
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Since memoir provides its own set of page layout macros, it's best to use them rather than resort to something like geometry. Also, using a4paper followed \setstocksize{297mm}{210mm} is superfluous, since a4paper has those dimensions exactly. In fact, if you choose the document class option a4paper with memoir, it executes \stockaiv, which is defined as

\newcommand*{\stockaiv} {\stockheight=297mm \stockwidth=210mm}

So, either use a4paperor\setstocksize{297mm}{210mm}, but not both.

In order to reduce the page size from 297mm x 210mm to 9in x 6in "while preserving the margin proportions and headers/footers", I would figure out the ratios between the two sizes knowing that

It kept the same proportions between the page size and the text block, but kept the trimming margins the same (15mm less than \stockwidth). The original is shown on the left, while the new is shown on the right at the same resolution (for comparison purposes):